The present invention relates to image-forming optical systems. More particularly, the present invention relates to a decentered optical system with a reflecting surface having a power for use in optical apparatus using a small-sized image pickup device, e.g. video cameras, digital still cameras, film scanners, and endoscopes.
Recently, with the achievement of small-sized image pickup devices, image-forming optical systems for use in video cameras, digital still cameras, film scanners, endoscopes, etc. have also been demanded to be reduced in size and weight and also in cost.
In the general rotationally symmetric coaxial optical systems, however, optical elements are arranged in the direction of the optical axis. Therefore, there is a limit to the reduction in thickness of the optical systems. At the same time, the number of lens elements unavoidably increases because it is necessary to correct chromatic aberration produced by a rotationally symmetric refracting lens used in the optical systems. Therefore, it is difficult to reduce the cost in the present state of the art. Under these circumstances, there have recently been proposed optical systems designed to be compact in size by giving a power to a reflecting surface, which produces no chromatic aberration, and folding an optical path in the optical axis direction.
Japanese Patent Application Unexamined Publication (KOKAI) Number [hereinafter referred to as xe2x80x9cJP(A)xe2x80x9d] 7-333505 proposes to reduce the thickness of an optical system by giving a power to a decentered reflecting surface and thus folding an optical path. In an example thereof, however, the number of constituent optical members is as large as five, and actual optical performance is unclear. No mention is made of the configuration of the reflecting surface.
JP(A) 8-292371, 9-5650 and 9-90229 each disclose an optical system in which an optical path is folded by a single prism or a plurality of mirrors integrated into a single block, and an image is relayed in the optical system to form a final image. In these conventional examples, however, the number of reflections increases because the image is relayed. Accordingly, surface accuracy errors and decentration accuracy errors are transferred while being added up. Consequently, the accuracy required for each surface becomes tight, causing the cost to increase unfavorably. The relay of the image also causes the overall volumetric capacity of the optical system to increase unfavorably.
JP(A) 9-222563 discloses an example of an optical system that uses a plurality of prisms. However, because the optical system is arranged to relay an image, the cost increases and the optical system becomes large in size unfavorably for the same reasons as stated above.
JP(A) 9-211331 discloses an example of an optical system in which an optical path is folded by using a single prism to achieve a reduction in size of the optical system. However, the optical system is not satisfactorily corrected for aberrations.
JP(A) 8-292368, 8-292372, 9-222561, 9-258105 and 9-258106 all disclose examples of zoom lens systems. In these examples, however, the number of reflections is undesirably large because an image is relayed in a prism. Therefore, surface accuracy errors and decentration accuracy errors of reflecting surfaces are transferred while being added up, unfavorably. At the same time, the overall size of the optical system unavoidably increases, unfavorably.
JP(A) 10-20196 discloses an example of a two-unit zoom lens system having a positive front unit and a negative rear unit, in which the positive front unit comprises a prism of negative power placed on the object side of a stop and a prism of positive power placed on the image side of the stop. JP(A) 10-20196 also discloses an example in which the positive front unit, which comprises a prism of negative power and a prism of positive power, is divided into two to form a three-unit zoom lens system having a negative unit, a positive unit and a negative unit. However, the prisms used in these examples each have two transmitting surfaces and two reflecting surfaces, which are all independent surfaces. Therefore, a relatively wide space must be ensured for the prisms. In addition, the image plane is large in size in conformity to the Leica size film format. Accordingly, the prisms themselves become unavoidably large in size. Furthermore, because the disclosed zoom lens systems are not telecentric on the image side, it is difficult to apply them to image pickup devices such as CCDs. In either of the examples of zoom lens systems, zooming is performed by moving the prisms. Accordingly, the decentration accuracy required for the reflecting surfaces becomes tight in order to maintain the required performance over the entire zooming range, resulting in an increase in the cost.
When a general refracting optical system is used to obtain a desired refracting power, chromatic aberration occurs at an interface surface thereof according to chromatic dispersion characteristics of an optical element. To correct the chromatic aberration and also correct other ray aberrations, the refracting optical system needs a large number of constituent elements, causing the cost to increase. In addition, because the optical path extends straight along the optical axis, the entire optical system undesirably lengthens in the direction of the optical axis, resulting in an unfavorably large-sized image pickup apparatus.
In decentered optical systems such as those described above in regard to the prior art, an imaged figure or the like is undesirably distorted and the correct shape cannot be reproduced unless the formed image is favorably corrected for aberrations, particularly rotationally asymmetric distortion.
Furthermore, in a case where a reflecting surface is used in a decentered optical system, the sensitivity to decentration errors of the reflecting surface is twice as high as that in the case of a refracting surface, and as the number of reflections increases, decentration errors that are transferred while being added up increase correspondingly. Consequently, manufacturing accuracy and assembly accuracy, e.g. surface accuracy and decentration accuracy, required for reflecting surfaces become even more strict.
In view of the above-described problems with the prior art, an object of the present invention is to provide a high-performance and low-cost image-forming optical system having a reduced number of constituent optical elements.
Another object of the present invention is to provide a high-performance image-forming optical system that is made compact and thin by folding an optical path using reflecting surfaces arranged to minimize the number of reflections.
The image-forming optical system according to the present invention provided to attain the above-described objects is an image-forming optical system having a positive refracting power as a whole for forming an object image. The image-forming optical system has a prism member formed from a medium having a refractive index (n) larger than 1 (n greater than 1). The prism member has a first entrance surface through which a light beam from an object enters the prism member. The prism member further has a first reflecting surface, a second reflecting surface, a third reflecting surface and a fourth reflecting surface, which reflect the light beam in the prism member. Further, the prism member has a first exit surface through which the light beam exits from the prism member. An optical path connecting the second reflecting surface and the third reflecting surface intersects an optical path connecting the first entrance surface and the first reflecting surface, and the optical path connecting the second reflecting surface and the third reflecting surface intersects an optical path connecting the fourth reflecting surface and the first exit surface. At least either one of the first reflecting surface and the second reflecting surface has a curved surface configuration that gives a power to a light beam. The curved surface configuration is a rotationally asymmetric surface configuration that corrects aberrations due to decentration. At least either one of the third reflecting surface and the fourth reflecting surface has a curved surface configuration that gives a power to a light beam. The curved surface configuration is a rotationally asymmetric surface configuration that corrects aberrations due to decentration. Moreover, an intermediate image plane is formed between the first reflecting surface and the fourth reflecting surface.
The reasons for adopting the above-described arrangement in the present invention, together with the function thereof, will be described below in order.
The image-forming optical system according to the present invention, which is provided to attain the above-described objects, has a positive refracting power as a whole for forming an object image. The image-forming optical system has a prism member formed from a medium having a refractive index (n) larger than 1 (n greater than 1). The prism member has a first entrance surface through which a light beam from an object enters the prism member. The prism member further has a first reflecting surface, a second reflecting surface, a third reflecting surface and a fourth reflecting surface, which reflect the light beam in the prism member. Further, the prism member has a first exit surface through which the light beam exits from the prism member. An optical path connecting the second reflecting surface and the third reflecting surface intersects an optical path connecting the first entrance surface and the first reflecting surface, and the optical path connecting the second reflecting surface and the third reflecting surface intersects an optical path connecting the fourth reflecting surface and the first exit surface.
A refracting optical element such as a lens is provided with a power by giving a curvature to an interface surface thereof. Accordingly, when rays are refracted at the interface surface of the lens, chromatic aberration unavoidably occurs according to chromatic dispersion characteristics of the refracting optical element. Consequently, the common practice is to add another refracting optical element for the purpose of correcting the chromatic aberration.
Meanwhile, a reflecting optical element such as a mirror or a prism produces no chromatic aberration in theory even when a reflecting surface thereof is provided with a power, and need not add another optical element only for the purpose of correcting chromatic aberration. Accordingly, an optical system using a reflecting optical element allows the number of constituent optical elements to be reduced from the viewpoint of chromatic aberration correction in comparison to an optical system using a refracting optical element.
At the same time, a reflecting optical system using a reflecting optical element allows the optical system itself to be compact in size in comparison to a refracting optical system because the optical path is folded in the reflecting optical system.
Reflecting surfaces require a high degree of accuracy for assembly and adjustment because they have high sensitivity to decentration errors in comparison to refracting surfaces. However, among reflecting optical elements, prisms, in which the positional relationship between surfaces is fixed, only need to control decentration as a single unit of prism and do not need high assembly accuracy and a large number of man-hours for adjustment as are needed for other reflecting optical elements.
Furthermore, a prism has an entrance surface and an exit surface, which are refracting surfaces, and a reflecting surface. Therefore, the degree of freedom for aberration correction is high in comparison to a mirror, which has only a reflecting surface. In particular, if the prism reflecting surface is assigned the greater part of the desired power to thereby reduce the powers of the entrance and exit surfaces, which are refracting surfaces, it is possible to reduce chromatic aberration to a very small quantity in comparison to refracting optical elements such as lenses while maintaining the degree of freedom for aberration correction at a high level in comparison to mirrors. Furthermore, the inside of a prism is filled with a transparent medium having a refractive index higher than that of air. Therefore, it is possible to obtain a longer optical path length than in the case of air. Accordingly, the use of a prism makes it possible to obtain an optical system that is thinner and more compact than those formed from lenses, mirrors and so forth, which are placed in the air.
In the present invention, a prism member comprising one or two decentered prisms is placed, and an object-side portion and image-side portion of the prism member are arranged to correct each other""s decentration aberrations, thereby enabling not only axial aberrations but also off-axis aberrations to be favorably corrected. If the number of reflections is one in each of the portions, it is impossible to correct decentration aberrations completely.
For the reasons stated above, the present invention is arranged so that a light beam is reflected four times in the prism member, and an intermediate image plane is formed in an optical path between the first reflecting surface and the fourth reflecting surface.
In the present invention, it is important that the axial principal ray should intersect itself twice in the prism (which may be a single integral prism or a combination of two separate prisms). If the axial principal ray substantially intersects itself twice in the optical system, the optical system can be folded so as to be compact in size.
The object-side portion of the prism member in the present invention has a first entrance surface, a first reflecting surface and a second reflecting surface and is arranged so that an optical path connecting the second reflecting surface and a third reflecting surface intersects an optical path connecting the first entrance surface and the first reflecting surface.
The prism object-side portion having such a configuration enables an increase in the degree of freedom for aberration correction and produces minimal aberrations. In addition, because the two reflecting surfaces of the prism object-side portion (i.e. the first reflecting surface and the second reflecting surface) can be positioned with a high degree of symmetry, aberrations produced by the two reflecting surfaces are corrected with these reflecting surfaces by canceling the aberrations each other. Therefore, the amount of aberration produced in the prism object-side portion is small. Furthermore, because the optical paths intersect each other in the prism object-side portion, the optical path length can be made long in comparison to a prism structure in which the optical path is simply folded. Accordingly, the prism object-side portion can be made compact in size, considering its optical path length. It is more desirable that the two reflecting surfaces in the prism object-side portion should have powers of different signs. By doing so, it is possible to enhance the effect of correcting each other""s aberrations by the two reflecting surfaces and hence possible to obtain high resolution.
In addition, if the prism object-side portion is formed by using a prism structure in which the optical paths intersect each other as stated above, it is possible to construct the prism object-side portion in a compact form. The reason for this is as-follows. In a comparison between the prism structure of the present invention and a prism structure of the same two-reflection type which has the same optical path length as that of the above-described prism structure and in which a Z-shaped optical path is formed, the prism structure of the present invention provides a higher space utilization efficiency. In the prism configuration having a Z-shaped optical path, rays invariably travel through different regions of the prism one by one, whereas in the prism in which the optical paths intersect each other, rays pass through the same region twice. Accordingly, the prism can be made compact in size.
The image-side (image-formation plane-side) portion of the prism member in the present invention has a third reflecting surface, a fourth reflecting surface and a first exit surface and is arranged so that the optical path connecting the second reflecting surface and the third reflecting surface intersects an optical path connecting the fourth reflecting surface and the first exit surface.
The arrangement of the prism image-side portion is similar to the arrangement of the above-described prism object-side portion. Thus, the prism image-side portion similarly enables an increase in the degree of freedom for aberration correction and produces minimal aberrations. In addition, because the two reflecting surfaces of the prism image-side portion (i.e. the third reflecting surface and the fourth reflecting surface) can be positioned with a high degree of symmetry, aberrations produced by the two reflecting surfaces are corrected with these reflecting surfaces by canceling the aberrations each other. Therefore, the amount of aberration produced in the prism image-side portion is small. Furthermore, because the optical paths intersect each other in the prism image-side portion, the optical path length can be made long in comparison to a prism structure in which the optical path is simply folded. Accordingly, the prism image-side portion can be made compact in size, considering its optical path length. It is more desirable that the two reflecting surfaces in the prism image-side portion should have powers of different signs. By doing so, it is possible to enhance the effect of correcting each other""s aberrations by the two reflecting surfaces and hence possible to obtain high resolution.
In addition, if the prism image-side portion is formed by using a prism structure in which the optical paths intersect each other as stated above, it is possible to construct the prism image-side portion in a compact form. The reason for this is as follows. In a comparison between the prism structure of the present invention and a prism structure of the same two-reflection type which has the same optical path length as that of the above-described prism structure and in which a Z-shaped optical path is formed, the prism structure of the present invention provides a higher space utilization efficiency. In the prism configuration having a Z-shaped optical path, rays invariably travel through different regions of the prism one by one, whereas in the prism in which the optical paths intersect each other, rays pass through the same region twice. Accordingly, the prism can be made compact in size.
Incidentally, in the present invention, the prism member may be formed from a combination of prisms cemented together or a single prism produced by integral molding. It is also possible to form the prism member from a combination of a first prism constituting the prism object-side portion and a second prism constituting the prism image-side portion.
When a light ray from the object center that passes through the center of the stop and reaches the center of the image plane is defined as an axial principal ray, if at least one reflecting surface of the object-side portion of the prism member in the present invention and at least one reflecting surface of the image-side portion of the prism member are not decentered with respect to the axial principal ray, the axial principal ray travels along the same optical path when incident on and reflected from each of the reflecting surfaces, and thus the axial principal ray is intercepted in the optical system undesirably. As a result, an image is formed from only a light beam whose central portion is shaded. Consequently, the center of the image is unfavorably dark, or no image is formed in the center of the image field.
In addition, at least either one of the first reflecting surface and the second reflecting surface of the object-side portion of the prism member in the present invention has a curved surface configuration that gives a power to a light beam, and the curved surface configuration is a rotationally asymmetric surface configuration that corrects aberrations due to decentration. Moreover, at least either one of the third reflecting surface and the fourth reflecting surface of the image-side portion of the prism member has a curved surface configuration that gives a power to a light beam, and the curved surface configuration is a rotationally asymmetric surface configuration that corrects aberrations due to decentration.
The reasons for adopting the above-described arrangements will be described below in detail.
First, a coordinate system used in the following description and rotationally asymmetric surfaces will be described.
An optical axis defined by a straight line along which the axial principal ray travels until it intersects the first surface of the optical system is defined as a Z-axis. An axis perpendicularly intersecting the Z-axis in the decentration plane of each surface constituting the imaging optical system is defined as a Y-axis. An axis perpendicularly intersecting the optical axis and also perpendicularly intersecting the Y-axis is defined as an X-axis. Ray tracing is forward ray tracing in which rays are traced from the object toward the image plane.
In general, a spherical lens system comprising only a spherical lens is arranged such that aberrations produced by spherical surfaces, such as spherical aberration, coma and curvature of field, are corrected with some surfaces by canceling the aberrations with each other, thereby reducing aberrations as a whole.
On the other hand, rotationally symmetric aspherical surfaces and the like are used to correct aberrations favorably with a minimal number of surfaces. The reason for this is to reduce various aberrations that would be produced by spherical surfaces.
However, in a decentered optical system, rotationally asymmetric aberrations due to decentration cannot be corrected by a rotationally symmetric optical system. Rotationally asymmetric aberrations due to decentration include distortion, curvature of field, and astigmatic and comatic aberrations, which occur even on the axis.
First, rotationally asymmetric curvature of field will be described. For example, when rays from an infinitely distant object point are incident on a decentered concave mirror, the rays are reflected by the concave mirror to form an image. In this case, the back focal length from that portion of the concave mirror on which the rays strike to the image surface is a half the radius of curvature of the portion on which the rays strike in a case where the medium on the image side is air. Consequently, as shown in FIG. 17, an image surface tilted with respect to the axial principal ray is formed. It is impossible to correct such rotationally asymmetric curvature of field by a rotationally symmetric optical system.
To correct the tilted curvature of field by the concave mirror M itself, which is the source of the curvature of field, the concave mirror M is formed from a rotationally asymmetric surface, and, in this example, the concave mirror M is arranged such that the curvature is made strong (refracting power is increased) in the positive direction of the Y-axis, whereas the curvature is made weak (refracting power is reduced) in the negative direction of the Y-axis. By doing so, the tilted curvature of field can be corrected. It is also possible to obtain a flat image surface with a minimal number of constituent surfaces by placing a rotationally asymmetric surface having the same effect as that of the above-described arrangement in the optical system separately from the concave mirror M.
It is preferable that the rotationally asymmetric surface should be a rotationally asymmetric surface having no axis of rotational symmetry in the surface nor out of the surface. If the rotationally asymmetric surface has no axis of rotational symmetry in the surface nor out of the surface, the degree of freedom increases, and this is favorable for aberration correction.
Next, rotationally asymmetric astigmatism will be described.
A decentered concave mirror M produces astigmatism even for axial rays, as shown in FIG. 18, as in the case of the above. The astigmatism can be corrected by appropriately changing the curvatures in the X- and Y-axis directions of the rotationally asymmetric surface as in the case of the above.
Rotationally asymmetric coma will be described below.
A decentered concave mirror M produces coma even for axial rays, as shown in FIG. 19, as in the case of the above. The coma can be corrected by changing the tilt of the rotationally asymmetric surface according as the distance from the origin of the X-axis increases, and further appropriately changing the tilt of the surface according to the sign (positive or negative) of the Y-axis.
The image-forming optical system according to the present invention may also be arranged such that the above-described at least one surface having a reflecting action is decentered with respect to the axial principal ray and has a rotationally asymmetric surface configuration and further has a power. By adopting such an arrangement, decentration aberrations produced as the result of giving a power to the reflecting surface can be corrected by the surface itself. In addition, the power of the refracting surfaces of the prism is reduced, and thus chromatic aberration produced in the prism can be minimized.
The above-described rotationally asymmetric surface used in the present invention should preferably be a plane-symmetry free-form surface having only one plane of symmetry. Free-form surfaces used in the present invention are defined by the following equation (a). It should be noted that the Z-axis of the defining equation is the axis of a free-form surface.                     Z        =                                            cr              2                        /                          [                              1                +                                                                            xe2x80x83                                                        ⁢                                      {                                          1                      -                                                                        (                                                      1                            +                            k                                                    )                                                ⁢                                                  c                          2                                                ⁢                                                  r                          2                                                                                      )                                                              ]                                +                                    ∑                              j                =                2                            66                        ⁢                          xe2x80x83                        ⁢                                          C                j                            ⁢                              X                m                            ⁢                              Y                n                                                                        (        a        )            
In Eq. (a), the first term is a spherical surface term, and the second term is a free-form surface term.
In the spherical surface term:
c: the curvature at the vertex
k: a conic constant
r=√{square root over ( )}(X2+Y2)
The free-form surface term is given by                                           ∑                          j              =              2                        66                    ⁢                      xe2x80x83                    ⁢                                    C              j                        ⁢                          X              m                        ⁢                          Y              n                                      =                ⁢                                            C              2                        ⁢            X                    +                                    C              3                        ⁢            Y                    +                                    C              4                        ⁢                          X              2                                +                                    C              5                        ⁢            XY                    +                                    C              6                        ⁢                          Y              2                                +                                    C              7                        ⁢                          X              3                                +                                    C              8                        ⁢                          X              2                        ⁢            Y                    +                                                ⁢                                            C              9                        ⁢                          XY              2                                +                                    C              10                        ⁢                          Y              3                                +                                    C              11                        ⁢                          X              4                                +                                    C              12                        ⁢                          X              3                        ⁢            Y                    +                                    C              13                        ⁢                          X              2                        ⁢                          Y              2                                +                                                ⁢                                            C              14                        ⁢                          XY              3                                +                                    C              15                        ⁢                          Y              4                                +                                    C              16                        ⁢                          X              5                                +                                    C              17                        ⁢                          X              4                        ⁢            Y                    +                                    C              18                        ⁢                          X              3                        ⁢                          Y              2                                +                                                ⁢                                            C              19                        ⁢                          X              2                        ⁢                          Y              3                                +                                    C              20                        ⁢                          XY              4                                +                                    C              21                        ⁢                          Y              5                                +                                    C              22                        ⁢                          X              6                                +                                    C              23                        ⁢                          X              5                        ⁢            Y                    +                                                ⁢                                            C              24                        ⁢                          X              4                        ⁢                          Y              2                                +                                    C              25                        ⁢                          X              3                        ⁢                          Y              3                                +                                    C              26                        ⁢                          X              2                        ⁢                          Y              4                                +                                    C              27                        ⁢                          XY              5                                +                                    C              28                        ⁢                          Y              6                                +                                                ⁢                                            C              29                        ⁢                          X              7                                +                                    C              30                        ⁢                          X              6                        ⁢            Y                    +                                    C              31                        ⁢                          X              5                        ⁢                          Y              2                                +                                    C              32                        ⁢                          X              4                        ⁢                          Y              3                                +                                    C              33                        ⁢                          X              3                        ⁢                          Y              4                                +                                                ⁢                                            C              34                        ⁢                          X              2                        ⁢                          Y              5                                +                                    C              35                        ⁢                          XY              6                                +                                    C              36                        ⁢                          Y              7                        ⁢            …                              
where Cj (j is an integer of 2 or higher) are coefficients.
In general, the above-described free-form surface does not have planes of symmetry in both the XZ- and YZ-planes. In the present invention, however, a free-form surface having only one plane of symmetry parallel to the YZ-plane is obtained by making all terms of odd-numbered degrees with respect to X zero. For example, in the above defining equation (a), the coefficients of the terms C2, C5, C7, C9, C12, C14, C16, C18, C20, C23, C25, C27, C29, C31, C33, C35, . . . are set equal to zero. By doing so, it is possible to obtain a free-form surface having only one plane of symmetry parallel to the YZ-plane.
A free-form surface having only one plane of symmetry parallel to the XZ-plane is obtained by making all terms of odd-numbered degrees with respect to Y zero. For example, in the above defining equation (a), the coefficients of the terms C3, C5, C8, C10, C12, C14, C17, C19, C21, C23, C25, C27, C30, C32, C34, C36, . . . are set equal to zero. By doing so, it is possible to obtain a free-form surface having only one plane of symmetry parallel to the XZ-plane.
Furthermore, the direction of decentration is determined in correspondence to either of the directions of the above-described planes of symmetry. For example, with respect to the plane of symmetry parallel to the YZ-plane, the direction of decentration of the optical system is determined to be the Y-axis direction. With respect to the plane of symmetry parallel to the XZ-plane, the direction of decentration of the optical system is determined to be the X-axis direction. By doing so, rotationally asymmetric aberrations due to decentration can be corrected effectively, and at the same time, productivity can be improved.
It should be noted that the above defining equation (a) is shown as merely an example, and that the feature of the present invention resides in that rotationally asymmetric aberrations due to decentration are corrected and, at the same time, productivity is improved by using a rotationally asymmetric surface having only one plane of symmetry. Therefore, the same advantageous effect can be obtained for any other defining equation that expresses such a rotationally asymmetric surface.
The image-forming optical system according to the present invention is an intermediate image formation type image-forming optical system in which an intermediate image plane is formed between the first reflecting surface and the fourth reflecting surface. By the first entrance surface, the first reflecting surface and the second reflecting surface, a light beam is rotated along a triangular path, thereby forming first intersecting optical paths. By the third reflecting surface, the fourth refecting surface and the first exit surface, a light beam is rotated along a triangular path, thereby forming second intersecting optical paths. The direction of rotation of the lights beam traveling along the triangular path to form the first intersection optical paths and the direction of rotation of the light beam traveling along the triangular path to form the second intersecting optical paths may be either the same or opposite to each other. The planes of rotation of the light beams may not extend parallel to each other but intersect each other, as a matter of course.
It is desirable that both the first reflecting surface and the second reflecting surface should have a curved surface configuration that gives a power to a light beam, and the curved surface configuration should be a rotationally asymmetric surface configuration that corrects aberrations due to decentration.
It is desirable for both the third reflecting surface and the fourth reflecting surface to have a rotationally asymmetric surface configuration that gives a power to a light beam and corrects aberrations due to decentration.
It is desirable for the first entrance surface to have a rotationally asymmetric surface configuration that gives a power to a light beam and corrects aberrations due to decentration.
It is desirable for the first exit surface to have a rotationally asymmetric surface configuration that gives a power to a light beam and corrects aberrations due to decentration.
In the above, it is desirable that the rotationally asymmetric surface configuration should be a plane-symmetry free-form surface having only one plane of symmetry.
In this case, the one and only plane of symmetry of the plane-symmetry free-form surface may be coincident with a plane formed by the axial principal ray traveling along the first intersecting optical paths.
The one and only plane of symmetry of the plane-symmetry free-form surface may be coincident with a plane formed by the axial principal ray traveling along the second intersecting optical paths.
The intermediate image plane may be formed between the second reflecting surface and the third reflecting surface.
In this case, it is desirable that the optical surfaces of the prism member that are closer to the object side than the intermediate image plane should be arranged to correct decentration aberrations as a whole and the optical surfaces of the prism member that are closer to the image-formation plane side than the intermediate image plane should be arranged to correct decentration aberrations as a whole so that the intermediate image plane is formed in an approximately planar shape.
Let us define the power of a decentered optical system and that of a decentered optical surface. As shown in FIG. 20, when the direction of decentration of a decentered optical system S is taken in the Y-axis direction, a light ray which is parallel to the axial principal ray of the decentered optical system S and which has a small height d in the YZ-plane is made to enter the decentered optical system S from the object side thereof. The angle that is formed between that ray and the axial principal ray exiting from the decentered optical system S as the two rays are projected onto the YZ-plane is denoted by xcex4y, and xcex4y/d is defined as the power Py in the Y-direction of the decentered optical system S. A light ray which is parallel to the axial principal ray of the decentered optical system and which has a small height d in the X-direction, which is perpendicular to the YZ-plane, is made to enter the decentered optical system from the object side thereof. The angle that is formed between that ray and the axial principal ray exiting from the decentered optical system S as the two rays are projected onto a plane perpendicularly intersecting the YZ-plane and containing the axial principal ray is denoted by xcex4x, and xcex4x/d is defined as the power Px in the X-direction of the decentered optical system S. The power Pyn in the Y-direction and power Pxn in the X-direction of a decentered optical surface n constituting the decentered optical system S are defined in the same way as the above.
Furthermore, the reciprocals of the above-described powers are defined as the focal length Fy in the Y-direction of the decentered optical system, the focal length Fx in the X-direction of the decentered optical system, the focal length Fyn in the Y-direction of the decentered optical surface n, and the focal length Fxn in the X-direction of the decentered optical surface n, respectively.
When the powers in the X- and Y-directions of the two reflecting surfaces (the first reflecting surface and the second reflecting surface) of the prism object-side portion that form intersecting optical paths are denoted by Px1-1, Py1-1, Px1-2 and Py1-2, respectively, in order from the object side, and the powers in the X- and Y-directions of the entire optical system are denoted by Px and Py, respectively, it is important to satisfy the following condition:
0.4 less than Px1-1/Px less than 1.1xe2x80x83xe2x80x83(1)
This condition defines the ratio of the power in the X-direction of the first reflecting surface to the power in the X-direction of the entire system. If Px1-1/Px is not larger than the lower limit, i.e. 0.4, the positive power of the first reflecting surface becomes excessively small, and it becomes necessary to assign a positive power to another surface. Consequently, the aberration correcting performance degrades. If Px1-1/Px is not smaller than the upper limit, i.e. 1.1, the positive power assigned to the first reflecting surface becomes excessively strong. Consequently, decentration aberrations produced by this surface become excessively large and hence difficult to correct by another surface.
It is even more desirable to satisfy the following condition:
0.6 less than Px1-1/Px less than 1.0xe2x80x83xe2x80x83(1-1)
Next, it is preferable to satisfy the following condition:
0.1 less than Px1-2/Px less than 0.6xe2x80x83xe2x80x83(2)
This condition defines the ratio of the power in the X-direction of the second reflecting surface to the power in the X-direction of the entire system. If Px1-2/Px is not larger than the lower limit, i.e. 0.1, the positive power of the second reflecting surface becomes excessively small, and it becomes necessary to assign a positive power to another surface. Consequently, the aberration correcting performance degrades. If Px1-2/Px is not smaller than the upper limit, i.e. 0.6, the positive power assigned to the second reflecting surface becomes excessively strong. Consequently, decentration aberrations produced by this surface become excessively large and hence difficult to correct by another surface.
It is even more desirable to satisfy the following condition:
0.1 less than Px1-2/Px less than 0.4xe2x80x83xe2x80x83(2-1)
When the powers in the X- and Y-directions of the two reflecting surfaces (the third reflecting surface and the fourth reflecting surface) of the prism image-side portion that form intersecting optical paths are denoted by Px2-1, Py2-1, Px2-2 and Py2-2, respectively, in order from the object side, and the powers in the X- and Y-directions of the entire optical system are denoted by Px and Py, respectively, it is preferable to satisfy the following condition:
0.2 less than Px2-1/Px less than 1xe2x80x83xe2x80x83(3)
This condition defines the ratio of the power in the X-direction of the third reflecting surface to the power in the X-direction of the entire system. If Px2-1/Px is not larger than the lower limit, i.e. 0.2, the positive power of the third reflecting surface becomes excessively small, and it becomes necessary to assign a positive power to another surface. Consequently, the aberration correcting performance degrades. If Px2-1/Px is not smaller than the upper limit, i.e. 1, the positive power assigned to the third reflecting surface becomes excessively strong. Consequently, decentration aberrations produced by this surface become excessively large and hence difficult to correct by another surface.
It is even more desirable to satisfy the following condition:
0.2 less than Px2-1/Px less than 0.8xe2x80x83xe2x80x83(3-1)
It is still more desirable to satisfy all the above-described conditions from the viewpoint of favorably correcting aberrations,
When the ratio of the power Px2-1 in the X-direction to the power Py2-1 in the Y-direction of the third reflecting surface is expressed by Px2-1/ Py2-1, it is preferable to satisfy the following condition:
0.5 less than Px2-1/Py2-1 less than 2.0 xe2x80x83xe2x80x83(4)
This condition defines the ratio of the power in the X-direction to the power in the Y-direction of the third reflecting surface. If Px2-1/ Py2-1 is not larger than the lower limit, i.e. 0.5, the power in the X-direction becomes excessively small with respect to the power in the Y-direction. Consequently, large astigmatic aberrations due to decentration occur. On the other hand, if Px2-1/Py2-1 is not smaller than the upper limit, i.e. 2.0, the power in the X-direction becomes excessively large with respect to the power in the Y-direction. Consequently, astigmatic aberrations due to decentration occur undesirably in the opposite direction.
It is even more desirable to satisfy the following condition:
0.5 less than Px2-1/Py2-1 less than 1.5xe2x80x83xe2x80x83(4-1)
In the image-forming optical system according to the present invention, focusing of the image-forming optical system can be effected by moving all the constituent elements or moving the prism. However, it is also possible to effect focusing by moving the image-formation plane in the direction of the axial principal ray exiting from the surface (the first exit surface) closest to the image side. By doing so, it is possible to prevent displacement of the axial principal ray on the entrance side due to focusing even if the direction in which the axial principal ray from the object enters the optical system is not coincident with the direction of the axial principal ray exiting from the surface closest to the image side owing to the decentration of the image-forming optical system. It is also possible to effect focusing by moving a plurality of wedge-shaped prisms, which are formed by dividing a plane-parallel plate, in a direction perpendicular to the Z-axis. In this case also, focusing can be performed independently of the decentration of the image-forming optical system.
In the present invention, temperature compensation can be made by forming the prism object-side portion and the prism image-side portion using different materials. By providing these prism portions with powers of different signs, it is possible to prevent the focal shift due to changes in temperature, which is a problem arising when a plastic material is used to form a prism.
In a case where the two prism portions are cemented together in the present invention, it is desirable that each of the two prism portions should have a positioning portion for setting a relative position on a surface having no optical action. In a case where two prism portions each having a reflecting surface with a power are cemented together as in the present invention, in particular, relative displacement of each prism portion causes the performance to be degraded. Therefore, in the present invention, a positioning portion for setting a relative position is provided on each surface of each prism portion that has no optical action, thereby ensuring the required positional accuracy. Thus, the desired performance can be ensured. In particular, if the two prisms are integrated into one unit by using the positioning portions and coupling members, it becomes unnecessary to perform assembly adjustment. Accordingly, the cost can be further reduced.
Furthermore, the optical path can be folded in a direction different from the decentration direction of the image-forming optical system according to the present invention by placing a reflecting optical member, e.g. a mirror, on the object side of the entrance surface of the image-forming optical system. By doing so, the degree of freedom for layout of the image-forming optical system further increases, and the overall size of the image-forming optical apparatus can be further reduced.
In the present invention, the image-forming optical system can be formed from a prism alone. By doing so, the number of components is reduced, and the cost is lowered. Furthermore, two prisms may be integrated into one prism with a stop put therebetween. By doing so, the cost can be further reduced.
In the present invention, the image-forming optical system may include another lens (positive or negative lens) as a constituent element in addition to the prism at either or each of the object and image sides of the prism.
The image-forming optical system according to the present invention may be a fast, single focal length lens system. Alternatively, the image-forming optical system may be arranged in the form of a zoom lens system (variable-magnification image-forming optical system) by combining it with a single or plurality of refracting optical systems that may be provided on the object or image side of the prism or between two prisms.
In the present invention, the refracting and reflecting surfaces of the image-forming optical system may be formed from spherical surfaces or rotationally symmetric aspherical surfaces, as a matter of course.
In the prism of the present invention, reflecting surfaces other than a totally reflecting surface are preferably formed from a reflecting surface having a thin film of a metal, e.g. aluminum or silver, formed on the surface thereof, or a reflecting surface formed from a dielectric multilayer film. In the case of a metal thin film having reflecting action, a high reflectivity can be readily obtained. The use of a dielectric reflecting film is advantageous in a case where a reflecting film having wavelength selectivity or minimal absorption is to be formed.
Thus, it is possible to obtain a low-cost and compact image-forming optical system in which the prism manufacturing accuracy is favorably eased.
In a case where the above-described image-forming optical system according to the present invention is placed in an image pickup part of an image pickup apparatus, or in a case where the image pickup apparatus is a photographic apparatus having a camera mechanism, it is possible to adopt an arrangement in which a prism member is placed closest to the object side among optical elements having an optical action, and the entrance surface of the prism member is decentered with respect to the optical axis, and further a cover member is placed on the object side of the prism member at right angles to the optical axis. The arrangement may also be such that the prism member has on the object side thereof an entrance surface decentered with respect to the optical axis, and a cover lens having a power is placed on the object side of the entrance surface of the prism member in coaxial relation to the optical axis so as to face the entrance surface across an air spacing.
If the prism member is placed closest to the object side and the decentered entrance surface is provided on the front side of a photographic apparatus as stated above, the obliquely tilted entrance surface is seen from the subject, and it gives the illusion that the photographic center of the apparatus is deviated from the subject when the entrance surface is seen from the subject side. Therefore, a cover member or a cover lens is placed at right angles to the optical axis, thereby preventing the subject from feeling incongruous when seeing the entrance surface, and allowing the subject to be photographed with the same feeling as in the case of general photographic apparatus.
A finder optical system can be formed by using any of the above-described image-forming optical systems according to the present invention as a finder objective optical system and adding an image-erecting optical system for erecting an object image formed by the finder objective optical system and an ocular optical system.
In addition, it is possible to construct a camera apparatus by using the finder optical system and an objective optical system for photography provided in parallel to the finder optical system.
In addition, an image pickup optical system can be constructed by using any of the foregoing image-forming optical systems according to the present invention and an image pickup device placed in an image plane formed by the image-forming optical system.
In addition, a camera apparatus can be constructed by using any of the foregoing image-forming optical systems according to the present invention as an objective optical system for photography, and a finder optical system placed in an optical path separate from an optical path of the objective optical system for photography or in an optical path split from the optical path of the objective optical system for photography.
In addition, an electronic camera apparatus can be constructed by using any of the foregoing image-forming optical systems according to the present invention, an image pickup device placed in an image plane formed by the image-forming optical system, a recording medium for recording image information received by the image pickup device, and an image display device that receives image information from the recording medium or the image pickup device to form an image for observation.
In addition, an endoscope system can be constructed by using an observation system having any of the foregoing image-forming optical systems according to the present invention and an image transmitting member for transmitting an image formed by the image-forming optical system along a longitudinal axis, and an illumination system having an illuminating light source and an illuminating light transmitting member for transmitting illuminating light from the illuminating light source along the longitudinal axis.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.